In the formation of transistors, metals were used for forming contact plugs and metal gates. Contact plugs are used for connecting to the source and drain regions and the gates of transistors.
In a conventional formation process for forming contact plugs, a first source/drain contact plug is formed in a first Inter-Layer Dielectric (ILD), and electrically connects to a source/drain region. A contact etch stop layer and a second ILD are then formed, and a gate contact opening is formed to extend into the second ILD, the contact etch stop layer, and the first ILD to expose the underlying metal gate. A source/drain contact opening is also formed to extend into the second ILD and the contact etch stop layer to expose the first source/drain contact plug. The gate contact opening and the source/drain contact opening are then filled with conductive materials to form a gate contact plug and a second source/drain contact plug. The resulting contact plugs formed in this process may suffer from voids formed therein, especially in the gate contact plug, which has a high aspect ratio.